by the way, 1500 memclock I've already tried and it made the screen flicker a lot, is something wrong with my card?
1500 is high for this memory and can cause artifacts... but because of the way timings are setup in the default BIOS, only 1250 or 1500 memclock will yield good performance.
You can try The STILT's 290 BIOS (works on most Tri-X cards) which changes the mem timings so that 1375-1450 will yield the same or better performance then 1500, but I would get your cards stable before flashing to his BIOS.
Basically tho, 1300 will be worse then 1250 on the stock BIOS, possible a lot worse.
I am using The Stilt's BIOS on my Tri-x cards, and am getting over 980KH/s on the best card (1400 mem 1100 engine), and over 920KH/s (1375 mem 1020 engine) on the worst card.